It has long been known to steam wood in order to soften the wood and improve its workability for veneer slicing. As an alternative to steaming, the wood can be soaked in hot water. The general opinion has been that the steamed or soaked log has absorbed large quantities of water and therefore has softened. However, recent research has shown that the wood absorbs but a few percent of water during steaming and soaking. What actually happens is that the lignin of the wood (the natural binder of the wood) is softened. One of the physical properties of lignin is that it softens at a temperature of about 65.degree. C. and more, depending upon the wood species. The steaming and soaking of wood brings several disadvantages, in spite of the fact that veneer producers have developed special techniques for controlling temperature curves etc. Soaked wood absorbs a certain amount of moisture, whereas steamed wood usually is dried out. Both methods produce stresses in the log which result in crack formation.
Naturally, it is desired to prevent such crack formation as far as possible, and also to affect the knots as little as possible in order to prevent the knots from falling out. A further disadvantage of the above-mentioned methods is the long heating time which is required in order to avoid too great a difference in temperature between the outer and inner parts of the log. As a result, the outer parts of the log will be subjected to heavy heat loads. A certain defibration (bursting of the wood cells) and leaching of lignin and rosin substances occurs, for which reason the veneer will be unnecessarily brittle and sensitive to further processing and handling. Furthermore, some species of wood are sensitive to steaming and soaking. Unless the pH is maintained at a value favourable to the wood, discoloration or other color changes may occur.
It has been attempted to slice veneer without heating the log. This is possible with thin veneer on a recently felled log under favourable conditions, but this technique subjects the veneer knife (the slicing tool) to hard wear and usually does not give a veneer of acceptable quality.
As is well known, there are two basic methods of producing veneer. The first method is by rotary cutting, which means that the log is clamped between two centers and rotated about its axis, while a knife is moved at a constant speed towards the log center, and more or less continuous veneer sheets are formed. By the other method, the so-called slicing method, the log is clamped on a bed, and a long knife slices a thin veneer sheet substantially transversely of the longitudinal direction of the log. When the knife returns to initial position, the log is advanced a distance corresponding to the veneer thickness. In a modification of this slicing method, the knife is stationary, while the log moves. Both the rotary cutting method and the slicing method require that the log is pretreated by heating.
Recently, another method of producing veneer has been developed, in which the log is moved longitudinally across an inclined knife, whereby veneer of desired thickness is obtainable. A reciprocating movement is imparted to the log, or the machine is provided with an additional conveying path for returning the log, and it is possible to have several logs in circulation at the same time. Also in this type of machine, the logs usually have been steamed or soaked.
If it is desired to prevent crack formation, steaming or soaking treatment may be replaced by the per se known technique which is used in drying wood and which implies that the water molecules within the wood are set in motion by electronic means, for instance by placing the log in an inductive or capacitative field. By suitably adapting the current and the voltage, a relatively uniform heating of the log can be achieved without any appreciable crack formation. The same effect is obtainable by placing the log in a field of microwaves. Both methods suffer from the disadvantage that the costs of installation are very high and that it is extremely difficult to maintain a homogeneous temperature throughout the log and to prevent drying-out of the log.